Ziegler-Natta catalyst system, which is comprised of a compound of Groups IV-VI transition metals of the Periodic Table as main catalyst and a compound of Groups I-III metals of the Periodic Table as co-catalyst, is wildly used in the polymerization of olefinic unsaturated monomers. The reaction mechanism and polymerization kinetics of Ziegler-Natta catalyst are recognized by scientists in the world. When Ziegler-Natta catalyst is used in the polymerization of ethylene, different polymerization processes require different polymerization kinetic behaviors of the catalyst, and the studies on polymerization kinetic behavior of the catalyst are therefore indispensable. In general, polymerization kinetic curve is used to characterize the kinetic behavior of a catalyst. Typical polymerization kinetic curves suitable for industrial production include: (1) slowly rising and slowly falling type; (2) quickly rising and damping type; (3) quickly rising and stable type; etc.
In a conventional gas-phase fluidized bed polymerization process, the removal of reaction heat is difficult and polymerization residence time is relatively long, so that a catalyst having a slowly rising and slowly falling type polymerization kinetic curve is suitable for this process. In a gas-phase fluidized bed polymerization process operated under condensing mode (CM), the ability of removing heat is greatly enhanced so that the production capacity increases, and the polymerization residence time is shorten to half of that of the conventional process. In such a process, a catalyst having a quickly rising and damping type polymerization kinetic curve is desired, because it can exhibit a relatively high activity in a short residence time and can therefore meet the technical requirements. In a slurry polymerization process, reaction heat can be sufficiently removed, and residence time is relatively long, so that a catalyst having a quickly rising and slowly falling type polymerization kinetic curve can be employed.
In CN1265609A, Mobil discloses a catalyst composition for the production of homopolymer and copolymer of ethylene, said catalyst composition comprising a procatalyst and a trialkylaluminum as co-catalyst, wherein said procatalyst comprises the following components; i) silica which is calcined at a temperature of at least 600° C.; ii) a dialkylmagnesium compound RmMgR′n; iii) a tetraalkyl orthosilicate; and iv) titanium tetrachloride, wherein the molar ratio of Ti/Mg is in a range of from 0.7 to 1.4. Said catalyst exhibits a relatively high activity in the polymerization of ethylene, and melt flow ratio (MFR) of the resulting polymer can be adjusted by altering the temperature for calcining the silica so that ethylene/1-hexene copolymers having different molecular weight can be obtained.
In CN1246808A, Mobil discloses a supported catalyst composition for the polymerization of α-olefin, said catalyst composition being prepared by a process comprising: impregnating an inorganic support having hydroxyl groups with a compound RMgR′ to form an intermediate; treating the intermediate with a halogen-containing reagent selected from tetrachloromethane and trichloroethane; contacting and reacting the treated intermediate with TiCl4 to form a procatalyst having a ratio of Mg to Ti of 0.5-2; and combining said procatalyst and a dialkylaluminum halide compound to form an α-olefin polymerization catalyst. Said halogen-containing reagent can effectively improve the activity of said catalyst. Said catalyst can be used in the polymerization or copolymerization of ethylene to produce a polymer with multimodal molecular weight distribution in a single reactor.
In CN1038754C, BP discloses a Ziegler-Natta catalyst with granular support and a process for preparing the same, said process comprising contacting and reacting a granular support with an organosilicone compound, such as diethoxydimethylsilane, a dialkylmagnesium or optionally a trialkylaluminum, a monochloro-organic compound, and at least one quadrivalent titanium compound. The obtained catalyst has a relatively high catalytic activity, and the polymer particles have better morphology, are of spherical shape or like-sphere shape, and have a bulk density of 0.37-0.50 g/cm3.
In EP0688794A, Borealis discloses an ethylene polymerization catalyst exhibiting a high activity and a better activity balance, i.e. exhibiting a balanced activity in the production of a high molecular weight polymer (having a low melt index) and a low molecular weight polymer (having a high melt index). Said catalyst is prepared by reacting an inorganic support, an alkyl metal chloride supported on said support, a magnesium compound, and a titanium halide, wherein the magnesium compound is obtained by reacting a dialkylmagnesium containing same or different alkyl groups with a mono-alcohol bearing a branched chain.
The catalysts as disclosed in the above documents are derived from dialkylmagnesium as starting raw material, and have different characteristics, such as high polymerization activity, balanced high polymerization activity, better particle morphology of the resultant polymers, and adjustable molecular weight distribution of the resultant polymers. However, these documents do not disclose or suggest that the kinetic behavior of said catalysts in ethylene polymerization can be controlled by altering the conditions for preparation of said catalyst.